1. Field of the Invention
A system and method for optimizing the output signal range of a photocell and more particularly, to a system and method of optimizing the output signal range of a photocell for all ambient lighting conditions by way of a serially connected switched resistor circuit, the duty cycle of which may be varied to provide an optimal signal range in both light and dark ambient lighting conditions.
2. Description of the Prior Art
Various types of photo sensors are known. One known type of photo sensor is a photocell. Such photocells act as light dependent resistors. In particular, the resistance output of such photocells varies as a function of the level of illumination incident on the surface of the device.
Such photocells are known to be made from Cadmium Sulfide ("CdS") Lead Sulfide ("PbS"), Cadmium Selenide, Ge, Si and other semiconductors. The resistance range of such photocells is known to vary from about 1-2 M ohms signal in darkness to as low as 10-1,000 ohms when the photocell is exposed to relatively bright light.
Various applications are known for such photocells. For example, photocells are widely used to control street lamps and other types of lamps in order to turn the lamps on at the onset of darkness and keep the lamp off during the daytime. Such photocells are also known to be used in automotive applications. In particular, in such automotive applications, the photocells are used to measure the glare relative to the ambient light level in order to control the reflectance of electrochromic mirrors used for rearview and sideview mirrors.
Because of the relatively wide resistance range of such photocells, matching resistors are known to be used to optimize the output signal range for night operation. In particular, such matching resistors are known to be serially connected to the photocells. The matching resistors are selected to provide an optimal signal range (i.e., resistance range) during nighttime conditions. Unfortunately, the resistance ranges of CdS photocells varies widely. Various techniques are used for calibrating such photocells, for example, as described in U.S. Pat. Nos. 5,193,029 and 5,451,822, hereby incorporated by reference. Calibration or matching resistors are known to be connected to the photocells to achieve a predetermined output level of the photocell for a predetermined light level. Such matching resistors are then hand-inserted into the assembly utilizing the photocell. Such a process is relatively labor-intensive and slows the manufacturing output time for manufacturing assemblies which include such photocells and also increases the price of the unit. There are other problems with such photocells. In particular, as mentioned above, a matched resistor is selected and serially connected to the photocell to optimize the output signal range during dark conditions. Once the matched resistor is hard-wired to the photocell, the output signal range of the photocell will not be optimal for measuring daytime light levels.